In the conventional way, the master cylinder is full of brake fluid and equipped with a main hydraulic piston intended to receive an actuating force composed of an input force and of a boost force both acting in an axial direction.
Also, the pneumatic booster can be controlled by the application of the input force to a control rod controlling the opening of a valve by means of a plunger in order to exert the actuating force on the main hydraulic piston of the master cylinder, the booster including a rigid casing divided in leaktight fashion into two chambers by means of a moving partition which can be acted upon by a difference in pressure between the two chambers resulting from the opening of the valve and can drive a pneumatic piston, which can move with respect to the casing, carrying the valve, and contributing at least to transmitting the boost force.
In braking systems with hydraulic reaction, the main hydraulic piston of the master cylinder itself includes a hollow moving cylinder communicating with the master cylinder and receiving at least some of the boost force, and inside which there slides, in leaktight fashion and in the axial direction, a reaction piston which can receive at least the input force, at least one opening being made in the moving cylinder to make the inside of the latter communicate with the inside of the master cylinder.
Such a device is described, for example, in document EP-B-0,443,886.
These devices with hydraulic reaction have as their advantages optimum control of the pressure of the brake fluid by the force exerted on the brake pedal and a characteristic operating curve giving the pressure prevailing in the master cylinder as a function of the input force exerted on the brake pedal, which is constant irrespective of the conditions of operation.
However, they have the drawback that when the reaction piston comes into contact with the plunger in order to transmit the reaction force to it, this contact is somewhat abrupt depending on the rate of application of the input force, and is transmitted to the control rod integral with the plunger and to the brake pedal articulated to the control rod and is felt as a rather unpleasant sensation by the driver of the vehicle. What is more, these impacts between the plunger and the reaction piston may damage the plunger and/or the reaction piston and detract from the correct operation of the braking system, or in any case reduce its life.